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Table of Contents

Introduction

Tools List

Background

Section 1: Preparations

Section 2: Opening the Cylinder

Section 3: Removing the Cylinder

Section 4: Replacing the Piston

Section 5: Reassembling the Cylinder

Section 6: Closing the Cylinder

Section 7: Preparing Motor for Regular Use

Appendix A: Motor Layout

Appendix B: Piston Schematic & Labeled Parts

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Cylinder Overhaul of a 1975 Yamaha DT125B Motorcycle

Disassembly, machining, and replacement of the cylinder unit

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Introduction:

Many problems with motorbike engines can be explained by a need to replace the piston

head and seals, and possibly re-boring the cylinder. Quite often, the user notices a lack of

power. The bike might not even start. Symptoms like these can be indicative of several

different problems. Problems with the fuel, ignition, or air intake systems could be to

blame. However, certain signs can point to the problem being within the cylinder itself.

For example, if the kick-starter moves more freely than it should, it could indicate

pressure loss in the engine. On bikes not equipped with a kick-starter, diagnosis might

require a more iterative system of checks. Is the fuel system working? Is the spark plug

working? Is the air intake plugged? If the problem cannot be found anywhere else, it may

be time to open the cylinder and inspect the piston seals. If the seals appear damaged, the

walls of the cylinder marred, or the piston itself deformed, an overhaul is required.

Please Note:

This document is dedicated to the process of disassembling and overhauling the cylinder

itself, and is not intended to be a diagnosis guide. Please refer to other guides, such as the

owners manual, or to professionals for diagnosis of the problem.

Recommended Tools List Philips Screwdriver (#2)

Flathead Screwdriver (1/4)

Needlenose Pliers

Razor Blade

Socket Wrench

17 mm Hex Socket

13 mm Hex Socket

12 mm Hex Socket

Shop Rags

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Background:

This section describes the physical processes and conditions that would require a cylinder overhaul.

This page can be skipped in the interest of time.

All motorcycle and ATV engines run on cylinders. Combustion occurs in the cylinder, which

converts thermal energy to mechanical energy. This mechanical energy is then transferred

through the gears to the wheels, moving the unit. The piston, within the cylinder, moves up and

down to convert the pressure to translational motion. The carburetor, attached to the cylinder,

forces fuel into the cylinder. A spark plug ignites the fuel. The exhaust system removes

combustion products from the engine. These components work together cyclically to power the

bike.

In older motors the gap between the cylinder walls and piston head can grow. This growth is

often caused by the piston seal wearing away, and can also be caused by deformation of the

cylinder head. As the gap between the cylinder walls and the piston head grows, more pressure

is lost from the engine. Pressure loss results in power loss, translating to decreased

performance. In severe cases, the engine may not even start.

When this gap becomes too large, the piston seals must be replaced. In severe cases, the cylinder

must be re-bored, meaning the walls of the cylinder must be machined to restore the surface

finish of the cylinder walls. Re-boring the cylinder requires that material be removed, resulting

in a slightly larger cylinder diameter, thus creating the need for a larger piston head.

Note: With proper maintenance of the engine, the pistons should not become deformed and the

cylinders not need to be re-bored. Abuse or neglect of the engine can result in the need for more

drastic repairs.

Figure 1: The basic two-stroke engine cycle. From left to right: (1), fuel and air are compressed into

the top end of the cylinder. At the same time, air and fuel (from the carburetor) are drawn into the

bottom of the cylinder.(2), the spark plug ignites the mixture. The rapid increase in temperature and

pressure forces the piston down. (3), as the piston nears the bottom of its stroke, the gases are

allowed to escape via the exhaust system. (4), new air and fuel are forced into the cylinder.

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Section 1: Preparations

A. Turn off the gas by turning the petcock and disconnect the tank from the carburetor. Remove the gas tank from the motorcycle frame in order to reduce the risk of gas spillage.

B. Take off spark plug cap.

Warning: If this first step is not completed, gas may leak from the tank and cause a safety hazard.

C. Remove exhaust by unscrewing bolts indicated below

D. Loosen the clamps on both ends of the carburetor and remove the bolts connect-ing to the cylinder.

E. Remove the carburetor from the air box and intake.

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Section 2: Opening the Cylinder

A. Loosen and remove cylinder casing bolts as indicated below. Note that there are 2 bolts which mirror those indicated that cannot be seen in the image below.

Tip: When removing bolts it is a good idea to bag and label them

B. Remove the head and head gasket from the cylinder in order to open the casing.

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Section 3: Removing the Cylinder

A. Bring piston down to its lowest point

with the kick starter.

B. Remove the cylinder from its casing.

This is how the piston looks at its lowest

point. Rotate the unit with the kick starter

until the cylinder stops moving downward.

If necessary, rotate the unit through an

entire cycle to get a feel for where the

bottom position is. This makes the cylinder

easier to remove.

This whole unit should lift off

assuming all other compo-

nents so far have been fully

removed.

C. Remove the intake and reed valves from

the cylinder.

Note: Following this section, you should take

the cylinder to a motorcycle shop to get it

bored. It is easier and cheaper to get the

existing cylinder bored than to get a brand

new cylinder. In addition, you will need to

purchase a new, larger piston to fit the new

cylinder.

These valves can be pulled directly out of the

unit with a pair of needle-nose pliers.

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Section 4: Replacing the Piston

A. Apply brake or carburetor cleaner to the

bored cylinder and wipe clean.

B. Stuff rag around rod in motor

C. Remove circlips (below left) on either

side of the old piston and push out the

wrist pin (below right) from the piston

joint.

D. Remove piston

E. Remove wrist pin bearing from rod.

Tip: This is to prevent debris from dropping

into the bottom end of the motor.

F. Remove old base gasket.

G. Make sure all gasket surfaces are clean

and smooth for new gaskets.

H. Put on new base gasket.

I. Put a new wrist pin bearing in the rod as

indicated in the below diagram (No. 5

below).

J. Put a circlip into one side of piston as

indicated (No. 4 below).

K. Place the piston (No. 2 below), with the

arrow on top of the piston facing

towards the exhaust port of the cylinder,

on top of the rod and push the wrist pin

through the piston and wrist pin bearing

until it hits previously inserted circlip.

L. Install 2nd circlip on the opposite site of

the wrist pin (No. 4 below).

M. Install new rings on the piston (No. 1

above) making sure the rings are lined

up with location pins that are in the

grooves of the piston.

Example of a piston being removed

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Section 5: Reassembling the Cylinder

A. Remove the rag from the bottom end of

the motor.

B. Coat the inside of the cylinder with 2-

stroke oil.

Tip: This is to lubricate and prevent metal

on metal friction.

C. Raise the piston to the top of the stroke

with the kick starter, as seen below.

Warning: Be careful not to pinch your fin-

gers during the next step.

D. Carefully lower the cylinder over the

piston while compressing the rings with

your fingers until the cylinder slides

down easily.

E. Make sure the piston moves up and

down the cylinder smoothly by careful-

ly holding the cylinder and pushing the

kick starter down.

Piston Down Piston Up

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Section 6: Closing the Cylinder

A. Install the new head gasket.

B. Install the head of the cylinder casing.

C. Install the four bolts into the head of the

cylinder casing and tighten the bolts.

D. Install the reads and intake with new

gaskets.

E. Replace the exhaust.

F. Replace the carburetor.

G. Replace the fuel tank on the bike frame.

H. Reattach the fuel line back to the

carburetor.

I. Replace the spark plug cap on the top of

the spark plug.

A gasket set for the motorcycle

Head gasket (not shown) goes on top of the

cylinder. Example of the top of a cylinder

shown above

An example of a cylinder intake without reeds, this is

where to you place the gasket. Note: This is where the

carburetor connects to the cylinder.

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Section 7: Preparing Motor for Regular Use

A. Start the motor and allow it to idle for

approximately 10 minutes.

B. Shut off the motor and allow it to fully

cool.

C. Repeat steps A-B three times.

D. Ride the bike at 1/2 throttle for 15

minutes.

E. Shut off the bike and allow it to fully cool.

F. Repeat steps D-E three times.

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Appendix A: Motor Layout

Left Side View (When sitting on the motorcycle)

Right Side View (When sitting on the motorcycle)

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1. Petcock: Can be used to cut off gas flow to carburetor

2. Spark plug Cap: Connects to spark plug, which is screwed into the cylinder head

3. Cylinder Head: Connected to the top of the cylinder

4. Carburetor: Connected to cylinder, lets in gas from gas tank

5. Cylinder: Where the piston is housed and where combustion takes place

6. Exhaust: Where the exhaust pipe is connected to the cylinder

7. Exhaust Pipe: Carries exhaust out of the cylinder

8. Kickstarter: Used to start the motorcycle

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Appendix B: Piston Schematic & Labeled Parts

1. Piston Rings

2. Piston Head

3. Wrist Pin (Piston Pin)

4. Circlips

5. Wrist Pin Bearing (Cylindrical Bearing)

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